The invention relates to a high-pressure sodium discharge lamp having a nominal power of at most 50 W provided with an elongate ceramic discharge vessel which has over a length L an at least substantially constant inner diameter .phi. and a substantially constant wall thickness d and in which electrodes are arranged with their tips opposite to each other at a relative distance D, each electrode being connected to a respective current supply conductor, which is passed to the exterior near an end of the discharge vessel, this lamp emitting during operation light having a color temperature of at least 2250K.
Such a lamp is known from British Patent Specification No. 2,083,281. The term "ceramic discharge vessel" is to be understood to mean a discharge vessel having a wall consisting of monocrystalline metal oxide (for example sapphire) or polycrystalline metal oxide (for example: densely sintered aluminium oxide, densely sintered aluminium-yttrium-garnet). The known lamp can be used to replace an incandescent lamp. The lamp emits during operation "white light", of which it holds for the color temperature Tc that 2250K.ltoreq.Tc.ltoreq.2750K and of which it holds for the general color index Ra.sub.8 that Ra.sub.8 .gtoreq.60. The region in the color triangle of the CIE chromaticity diagram within which the light of a high-pressure discharge lamp is designated as "white" is limited by straight lines through points with coordinates (x,y): (0.468; 0.430), (0.510; 0.430), (0.485; 0.390) and (0.445; 0.390). More stringent standards based on a value of the general color index Ra.sub.8 .gtoreq.75 correspond in the color triangle to the region enclosed by the lines x=0.468, x=0.490, y=0.408 and y=0.425.
Lamps of this kind are suitable to replace incandescent lamps because of their longer life and their considerably higher efficiency.
In case the lamp replacing an incandescent lamp is used in a reflector luminaire, it is necessary that the optical dimensions of the discharge arc, viewed through the wall of the discharge vessel, are at least of substantially the same size as the dimensions of the helical filament of the replaced incandescent lamp. When the dimensions of the discharge arc of the known lamp are further reduced, this has the disadvantage that the efficiency of a lamp of this kind decreases or that the lamp is overloaded, as a result of which the life is shortened.